The adenoviruses (Ads) comprise a large family of double-stranded DNA viruses found in amphibians, avians, and mammals which have a nonenveloped icosahedral capsid structure (Straus, Adenovirus infections in humans; The Adenoviruses, 451-498, 1984; Hierholzer et al., J. Infect. Dis., 158: 804-813, 1988; Schnurr and Dondero, Intervirology., 36: 79-83, 1993; Jong et al., J. Clin. Microbiol., 37: 3940-3945: 1999). In contrast to retroviruses, adenoviruses can transduce numerous cell types of several mammalian species, including both dividing and nondividing cells, without integrating into the genome of the host cell.
Generally speaking, adenoviral DNA is typically very stable and remains episomal (e.g., extrachromosomal), unless transformation or tumorigenesis has occurred. In addition, adenoviral vectors can be propagated to high yields in well-defined production systems which are readily amenable to pharmaceutical scale production of clinical grade compositions. These characteristics and their well-characterized molecular genetics make recombinant adenoviral vectors good candidates for use as vaccine carriers. The production of recombinant adenoviral vectors may rely on the use of a packaging cell line which is capable of complementing the functions of adenoviral gene products that have been either deleted or engineered to be nonfunctional.
Presently, two well-characterized human subgroup C adenovirus serotypes (i.e., hAd2 and hAd5) are widely used as the sources of the viral backbone for most of the adenoviral vectors that are used for gene therapy. Replication-defective human adenoviral vectors have also been tested as vaccine carriers for the delivery of a variety of immunogens derived from a variety of infectious agents. Studies conducted in experimental animals (e.g. rodents, canines and nonhuman primates) indicate that recombinant replication-defective human adenoviral vectors carrying transgenes encoding immunogens as well as other antigens elicit both humoral and cell-mediated immune responses against the transgene product. Generally speaking, investigators have reported success using human adenoviral vectors as vaccine carriers in nonhuman experimental systems by either using immunization protocols that utilizes high doses of recombinant adenoviral vectors that are predicted to elicit immune responses; or by using immunization protocols which employ the sequential administration of adenoviral vectors that are derived from different serotypes but which carry the same transgene product as boosting immunizations (Mastrangeli, et. al., Human Gene Therapy, 7: 79-87 (1996).
Viral vectors based on human adenovirus type 5 (Ad5) have been developed for different gene therapy and vaccine applications. Although Ad5-based vectors are extremely efficient in animal models, the presence of a pre-existing immunity in humans against Ad5 wild type virus has been demonstrated in clinical trials to reduce the efficiency of gene transduction. In particular, a clear reduction of the immunization efficiency was demonstrated in subjects with titers of neutralizing antibodies over 200 enrolled in vaccine clinical trial based on Ad5 vectors. The most extensive characterization of an Ad5 vectored vaccine was obtained in the HIV vaccine STEP trial conducted by Merck (Moore J P et al. Science. 2008 May 9; 320(5877):753-5). The vaccine study was based on the co-injection of 3 Ad5 vectors expressing different HIV antigens as proof of concept study in subjects with high risk of HIV infection. Surprisingly, the data revealed an increase of HIV infection rate in vaccinated subjects with anti-Ad5 pre-existing immunity rather then a protective effect. Although the mechanism of this paradoxical observation is not clear yet, the results raised additional questions on the safety and efficiency of vectors based on adenovirus of human origin for vaccine application in healthy subjects. Taken together all results obtained so far in different vaccine and gene therapy clinical trials such as the trials with Ad5 vectors increased the need for an adenovirus characterized in a very low or absent pre-existing immunity in humans.